Briefly, this invention has to do with an improvement in the plug-in fuse assembly disclosed in U.S. Pat. No. 3,909,767, granted Sept. 30, 1975, and which preferably comprises a plug-in element including a plate-like body of fuse metal having a pair of spaced confronting terminal blade portions to be received by pressure clip terminals in a mounting panel, current-carrying extensions at the inner end portions of the pair of terminal blade portions and a fuse link portion of reduced thickness interconnecting the current-carrying extensions. An insulating body, preferably forming a synthetic plastic housing, is anchored between the current-carrying extensions, with the pair of terminal blade portions thereof extending outwardly from the housing. One of the cost saving and size reducing aspects of the plug-in fuse assembly just described is that each plug-in fuse element can be a stamping made from a blank or strip of fuse metal, and a completely housed fuse results from merely enclosing the same in an insulating housing, so that the entire fuse assembly is formed of only two parts, and without any soldering operations required to connect a fuse link between the terminal portion of the fuse. For electrical testing purposes, the outer wall of the hosuing facing away from the side of the housing from which the terminal blade portions extend may be provided with probe-receiving apertures into or adjacent to which the outer end portions of the current-carrying extensions extend. These apertures receive test probes for testing the resistance or continuity of the fuse link portion of the plug-in fuse element. The fuse link portion of the plug-in fuse element is shown as a straight link positioned in spaced but contiguous relation to this outer wall of the housing which was transparent, so that the fuse link portion could be readily viewed through this outer wall when a number of plug-in fuse assemblies are mounted in close juxtaposed relation in a mounting panel where the sides of the housing are not readily visible. The housing is shown formed to receive the current-carrying extensions of the plug-in fuse element in relatively closely spaced relation. The housing walls are preferably staked into anchoring apertures in the current-carrying extensions of the plug-in fuse element to anchor the same thereto.
The above-described plug-in fuse assembly disclosed in this patent, while satisfactory for intermediate current ratings, causes problems at low or high current ratings. The high current fuse problem is that the plastic housing sometimes melted before the fuse blew. It was discovered that this resulted, in part, from the fact that the anchoring apertures were positioned centrally in the current-carrying extensions where they are located between the point of intersection of the fuse link portion of the plug-in fuse element and the terminal blades, so that current flowing between the fuse link and the terminal blades must pass through a portion of the current-carrying extensions reduced in cross-sectional area by the anchoring apertures. The temperature of this section of the plug-in fuse element next to the housing can be appreciable at high current flow because the resistance to current flow is inversely proportional to the cross-sectional area of the current path and the heat developed in a resistance is proportional to the square of the current flow. There is, therefore, a danger that a housing made of a plastic material will be damaged at the point where the anchoring apertures are located.
In accordance with one of the features of the invention, for a relatively high fuse rating (for example of 25 and 30 amp fuses), the fuse link is located on the terminal blade side of the anchoring apertures so load current does not flow through the reduced portion of the current-carrying extensions. Also, heat dissipation is increased by preferably locating the point where the fuse link joins the current-carrying extensions close to the terminal blade end of the current-carrying extensions so that the heat sink effect of the pressure clip terminals is increased by bringing the medium current path point closer thereto and the total path resistance for current flow is decreased.
Another difficulty with the plug-in fuse assembly design shown in said patent is that a low current ratings using the straight fuse link, the fuse link sometimes collapsed before the current reached a level where it was supposed to melt or soften appreciably. To obtain the high resistance necessary to generate the heat necessary to melt the fuse link at rated current, with a short straight fuse link the cross-sectional area thereof was reduced to such a small value that while the fuse link was self-supporting at normal low current levels, currents approaching the fuse rating will weaken the fuse link to a point of collapse even though the fuse link did not yet melt or soften appreciably.
In accordance with another feature of the invention, instead of providing a fuse link which is formed by a straight section of fuse metal when the fuse link must have such a cross-sectional area that it collapses when the temperature thereof approaches but does not reach the melting point of the fuse, a more substantial safe cross-sectional area is utilized and the necessary resistance is provided at the lowest current ratings by a repeatedly undulating fuse link configuration to provide a relatively large resistance for a given limited spacing between the current-carrying extensions provided for the fuse link. The major portion of the length of such an undulating configuration extends generally parallel to the length of the current-carrying extensions. Also, to minimize the heat sinking effect of the pressure clip terminals which receive the terminal blade portions of the plug-in fuse element, at least one end of the fuse link makes contact with a current-carrying extension at the end thereof most remote from the end adjacent to the terminal blade portions thereof, where the heat sink effect of the fuse support is minimized.
Further objects of this invention reside in the particular method steps and in the cooperative relationships between the method steps in making the aforementioned plug-in fuse assembly.
The previously described and other features of this invention will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawings.